Various electronic apparatuses such as a printer, a copying machine, a keyboard and an in-vehicle machine are equipped with a circuit board having electronic parts mounted thereon. With the increasing demand for lightness, thinness, shortness and smallness of the electronic apparatus, the wiring pattern of the circuit board is highly dense and thinned. Thus, static electricity discharged from a user may be discharged to signal lines of the circuit board. If the static electricity is discharged to the signal lines, a high voltage may be instantaneously applied to thereby break the electronic parts mounted on the circuit board.
JP-A-5-41568 discloses a configuration in which one set of discharge stylus patterns form air gaps between an earth pattern and input/output terminals. A mounting pattern, on which a resistor chip is mounted, is formed between input/output terminals and electronic parts. The distance of the air gaps formed by the discharge stylus patterns is set smaller than that of the mounting pattern of the resistor chip. Thus, the static electricity is discharged to the air gaps and released to the earth pattern and is prevented from being discharged to the mounting pattern of the resistor chip.
JP-A-10-270812 discloses a configuration, in which an insulated silk is printed on a pair of wiring patterns and on a solder resist layer arranged between the pair of wiring patterns. By this configuration, the dielectric strength is improved without widening the pattern gap between the pair of wiring patterns. Thus, the discharge to the wiring patterns can be prevented and the size of the circuit board can be reduced.